1. Field of the Invention
This invention relates to the field of optically examining sheets of paper, such as bank notes, that are not very transparent. In particular, it relates to a device for dynamically measuring the amount of dirt accumulated on a bank-note.
2. The Prior Art
In connection with the continuing tendency of employing automatic methods in the fields of financial services, efforts are also made to an increasing extent for finding reliable and automatically analizable criteria for detecting whether the bank-notes in circulation are up to the standards of usefulness.
One criterion for measuring the or unusefulness of a bank-note, for example, is the degree to which the bank-notes are soiled.
In relation to the foregoing, a device has already become known (German Published Patent Application (DT-OS) No. 2,156,077), corresponding to U.S. Pat. No. 3,718,823 in which the light directed through the edge of a bank-note, impinges upon a scanning sensor. In a comparator the output signal of the scanning sensor which is dependent upon the transparency and, consequently, upon the soiling or degree of dirt accumulation on the edge of the bank-note, is compared with a reference signal corresponding to a non-soiled edge of a bank-note. The output signal of the comparator is used for deciding on whether the banknote is to be regarded as soiled or not.
Of a disadvantage in this process is the fact that owing to the principle of measurement, not only the soiling, but also the transparency is considered in the measuring signal.
Investigations, however, have shown that unused and used, as well as a variety of new bank-notes all have different transparencies. Therefore, the more the transparency differs, the greater the range of tolerance of the comparison signal has to be chosen, which considerably reduces the accuracy of the measurement and, consequently, the reproducibility of the once achieved degree of sorting out. The same also applies to the relatively strongly varying average shading component of the paper used for manufacturing the bank-notes. Moreover, also the high investment involved in circuitry needed for producing and storing a reference signal bearing the scanning contents of a reference bank-note is considered as a disadvantage.